Damper

Damper Applications GuideConstant Volume Damper and Actuator Characteristics 55.33

Actuators for heating, ventilation, air-conditioning

SIGNALFA

FS

SPRINGDIAPHRAGM

PARALLEL BLADEDAMPER

OPPOSED BLADEDAMPER

FAN

FAN

ACTUATORS

EA

OA

RA

SAHeating/CoolingEtc.

RA

10 20 30

10

20

30

0

VARIATION50 - 150%

OF DESIREDMIN. OAVOLUME

HYSTERESIS = 12%

OAFLOW

%

% OA DAMPER POSITION

Detail of an Opposed Blade Damperwith an authority of 10%*

MIN. 10% OA FLOW DESIRED5 - 15% RESULT

CONTROL POINT = 6 OF DESIRED MIN.OA DAMPER POSITION

0 10 20 30 40 50 60 70 80 90

100

90

80

70

60

50

40

30

20

10

0

MAXIMUM

DEGREES, DAMPER ROTATION

PER

CENT

O

F M

AXIM

UM T

ORQ

UE

TYPICAL TORQUE REQUIREMENT

NEGATIVE POSITIVE

0 10 20 30 40 50 60 70 80 90 100

FLOW

%

100

90

80

70

60

50

40

30

20

10

0

TOTAL FLOWMA

RA

OA

IDEAL FUNCTION

OA/RA MIXING RATIO

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INDEX

I. INTRODUCTIONA. Economizer Systems ............................................................................................................ 3B. Indoor Air Quality ...................................................................................................................3C. Damper Selection ..................................................................................................................3

II. DAMPER SELECTIONA. Opposed and Parallel Blade Dampers ..................................................................................3B. Flow Characteristics...............................................................................................................4C. Damper Authority ..................................................................................................................4D. Combined Flow ......................................................................................................................6

III. ADDITIONAL CONSIDERATIONSA. Economizer Systems .............................................................................................................6B. Building Pressure ...................................................................................................................8C. Mixed Air Temperature Control ..............................................................................................8D. Actuators - General ................................................................................................................8E. Hysteresis ..............................................................................................................................9F. Accuracy ...............................................................................................................................9G. Actuators - Pneumatic..........................................................................................................10H. Positioners ...........................................................................................................................11I. Torque..................................................................................................................................11J. Linkage.................................................................................................................................11K. Actuators - Electronic ...........................................................................................................12L. Electronic Actuators - Conventional Crank Arm Type..........................................................12M. Electronic Actuators - Direct Coupled ..................................................................................13

IV. OPTIMIZING ECONOMIZER SYSTEMSA. Sizing of Dampers ...............................................................................................................14B. Linearization.........................................................................................................................15C. Range Controller ..................................................................................................................15D. Choice of Dampers ..............................................................................................................16

V. ADVANTAGESA. Direct Coupled Actuators .....................................................................................................17B. Range Controller ..................................................................................................................17

VI. MODERNIZATION OF OLD INSTALLATIONS.............................................................................................................................................17

VII. SUMMARY.............................................................................................................................................17

REFERENCES Air Movement and Control Association, Inc. (AMCA) Publication 502-89

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE)1991 Applications Handbook, sec. 41.6

We would like to thank Bengt Carlson, Energy Management Consultant, the primary contributor to this book, along with Larry Felker, Area Sales Manager, and RobertBalkun, Product Manager, for their efforts in creating this Application Guide.

If you have any comments on the information given or possible additions, please send them to Robert Balkun, Product Manager, Belimo Aircontrols (USA), Inc., P.O. Box2928, Danbury, CT 06813, (800) 543-9038, in CT (203) 791-9915, FAX (203) 791-9919.The ideas and concepts shown in this book are based on standard, HVAC engineering guide lines. The use of the information given must be used with sound engineeringjudgement. Belimo Aircontrols does not take responsibility for problems caused by the concepts given in this publication.All the contents of this publication are the sole property of Belimo Aircontrols (USA), Inc. and may not be reproduced without the written authorization of Belimo Aircontrols(USA), Inc. 1999 Belimo Aircontrols (USA), Inc.

Control of Dampers

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Damper Applications Guide 1

I. INTRODUCTIONI-A. Economizer Systems

The economizer portion of an air-handling system is not only responsi-ble for reducing the operating cost ofthe system, but more importantly, ithas to provide a sufficient volume ofoutside air at all times to ensuregood indoor air quality. The functionof the economizer also affects thepressurization of the building. Aslight positive pressure is needed toprevent infiltration of unconditionedoutside air. Otherwise the indoor cli-mate can be adversely affected dueto draft and cold perimeter walls.This is typically compensated for by increasing the room temperature, which increases the operating cost. However, too high pres-sure causes exfiltration, which increases the heat losses. Also, an incorrect pressurization may cause damages to the building struc-ture due to condensation, freezing etc. See Fig. 1.

I-B. Indoor Air Quality

The indoor air quality and comfort is much more important than the operating cost of the air handling system. If it is not adequate,absenteeism and turnover will increase and productivity will suffer. This is a hidden cost, which is hard to quantify, but nevertheless,it is a very large cost that can seriously affect the bottom line of any organization. Just imagine what the cost of a few percentdecrease in the productivity can be. While the cost of air conditioning typically is $2/sq. ft., the cost of productive workers is $150/sq. ft.

The ventilation system is the cause of almost half the indoor air quality complaints according to NIOSH (National Institute ofOccupational Safety and Health). Damper control alone is itself not responsible for the indoor air quality and climate, but it has avery central role. Therefore, it is important that the dampers and actuators are selected, sized and operated correctly.

I-C. Damper Selection

Traditionally, little attention is given to the selection of the dampers that comprise the economizer. What is even worse is that theselection of the actuators that operate the dampers is based upon one criterion only, and that is that they provide sufficient torque tooperate the dampers. The accuracy of the control is rarely considered. Without accurate damper positioning, the economizer willnot function correctly, and both the indoor air quality and comfort is compromised.

The following text discusses the importance of selecting and sizing the dampers for correct operation.

II. DAMPER SELECTIONII-A. Opposed and Parallel BladeDampers.

In HVAC installations two differenttypes of rectangular dampers areused to modulate air flow. These areparallel and opposed blade dampers.

Parallel blade dampers are con-structed so all the blades move in thesame direction and in parallel. SeeFig 2.

Opposed blade dampers are con-structed so blades next to each othermove in opposite directions. See Fig. 3.

3

FAN

FAN

ACTUATORS

EA

OA

RA

SAHeating/CoolingEtc.

RA

Fig. 1 - Economizer System

PARALLEL BLADEDAMPER

OPPOSED BLADEDAMPER

Fig. 2 Fig. 3

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II B. Flow Characteristics

The two types of dampers have dif-ferent inherent flow characteristics.This can be illustrated by a curvethat shows the relationship betweenthe flow rate and the position of theblades.

Fig. 4 shows the inherent flow char-acteristics for both parallel and opposedblade dampers. Notice that neitherdamper has a linear characteristic.

Opposed blade dampers give a veryslow increase in the flow when thedamper begins to open.

Parallel blade dampers have an inher-ent curve that is not as pronounced,so the flow increases more rapidlywhen the damper begins to open.

It is very important to realize that theinherent curves are measured underlaboratory conditions, with a constantdifferential pressure across thedamper. (In a real installation thecurves will be distorted. See Fig. 6and 7.)

II-C. Damper Authority

When installed in a system, thedamper is not the only device whichaffects the flow. Other parts of thesystem, such as the duct work, fil-ters, coils, etc., will also restrict theflow. The result is that the damperwill not control the flow in accordance toits inherent flow characteristics.

When the damper is closed (See Fig.5A), the full differential pressure willfall across the damper, but when it isopen (See Fig. 5B) the pressure dropacross all other parts will be large,leaving little pressure drop acrossthe damper.

The pressure drop across thedamper changes as the damper isoperated from closed to open. Thisdistorts the f low characteristicscurve. When the damper begins toopen, the differential pressure ishigh, and the flow increases at ahigher rate than what the inherentcurve suggests. When the damper isalmost fully open, the pressure dropacross the damper will be small,

4


Fig. 4 - Inherent Characteristics of a Parallel and Opposed Blade Dampers

Fig. 5A

FILTER

COIL

COIL

DAMPERPRESSURE DROP = 1.0 IN WG

TOTAL SYSTEM PRESSURE DROP = 1.0 IN WG

CLOSEDDAMPER

NO FLOW

Fig. 5B

FILTER

COIL

COIL

OPEN DAMPERPRESSURE DROP = .1 IN WG

TOTAL SYSTEM PRESSURE DROP = 1.0 IN WG

OPENDAMPER

LARGE FLOW

AUTHORITY % =.1

1.0= 10%

0 10 20 30 40 50 60 70 80 90

FLOW

%

100

90

80

70

60

50

40

30

20

10

0

NOTE:The characteristics will be different when installed in a system. See Fig. 6.

POSITION

PARALLEL BLADE DAMPER

0 10 20 30 40 50 60 70 80 90

FLOW

%

100

90

80

70

60

50

40

30

20

10

0

NOTE:The characteristics will be different when installed in a system. See Fig. 7.

POSITION

OPPOSED BLADE DAMPER

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compared to the total pressure drop,so if the damper moves, for example,from 80 to 90 opening, the flow willchange very little.

A damper which is sized so it has asmall resistance when fully open incomparison to the total system resis-tance, will have a small influence(authority) on the flow when it oper-ates near the fully open position.

The resistance of the fully opendamper can be expressed as a per-centage of the total system resis-tance and is called DamperAuthority or Characteristic Ratio.

Damper Authority % =Open Damper Resistance x 100%Total System Resistance

The resistance is a pressure drop, sowe can also write:Open Damper Pressure Drop x 100%Total System Pressure Drop

The pressure variation across thedamper distorts the inherent charac-teristics curves. Instead, we will getan Installed Damper Characteristic,which depends upon the DamperAuthority. See Fig. 6 and 7. If theauthority is 100%, then we will getthe inherent characteristic, but thiswill rarely happen in a real installa-tion. Instead, a much lower number(Ex. 1 - 10%) is realistic. As can beseen in Fig. 6 and 7, at an authorityof 1%, the curve deviates greatlyfrom the inherent curve (100%).

It is very important to realize that theTotal System Resistance or TotalSystem Pressure Drop, only relatesto the part of the system where theflow is controlled by the damper.See Fig. 8. The flow downstream ofthe mixing plenum (point A) is notcontrolled by the OA damper (it isconstant, at least in theory). The OAdamper only controls the f lowthrough the weather louver, thedamper and the OA duct work.Therefore the authority of the OAdamper is determined by the pressure drop across the OA damper as a percentage of the total differential pressure between OA and point A.

The total system pressure drop for the OA damper is measured between OA and Point A. The RA damper is measured betweenPoint B and Point A. The EA damper is measured between Point B and EA.

If a single damper is used to control the flow through an air handler and duct work, then the total system pressure drop is the sum ofthe pressure drops in all the parts. (See Fig. 5A and B)

5

0 10 20 30 40 50 60 70 80 90

100

90

80

70

60

50

40

30

20

10

0

INSTALLED FLOW CHARACTERISTICS AT DIFFERENT DAMPER AUTHORITIES (1-100%)

DAMPER POSITION, DEGREES OPEN

PERC

ENT

OF M

AXIM

UM F

LOW

2 3 4 5 6 7 8 9 10VCONTROL SIGNAL

1%2%

3%4%

5%10%

15%

20%

30%

50%

100%

Fig. 6 - Parallel Blade Damper Flow Characteristics

100

90

80

70

60

50

40

30

20

10

0

PERC

ENT

OF M

AXIM

UM F

LOW


1%2%

3%4%

5%

10%

15%

20%30%

50%

100%

0 10 20 30 40 50 60 70 80 90 DAMPER POSITION, DEGREES OPEN

2 3 4 5 6 7 8 9 10VCONTROL SIGNAL

Fig. 7 - Opposed Blade Damper Flow Characteristics

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II-D. Combined Flow

The sizing of the OA, RA and EAdampers installed in an Economizeris very important because it deter-mines the installed damper charac-teristics. If the dampers have beenproperly sized, their characteristicswill complement each other so thetotal flow will be constant. See Fig.9. The pressure at points A and Bwill remain constant regardless of theOA/RA mixing ratio.

See Fig. 10. The OA damper modu-lates the OA flow. When it is fullyopen, a large portion of the differen-tial pressure wil l fal l across theweather louver and the OA ductwork, resulting in a relatively smalldifferential pressure across the OAdamper. When the OA damper iscompletely closed, the full differentialpressure wil l fal l across the OAdamper. The authority of the OAdamper is the differential pressureacross the damper when it is fullyopen, as a percentage of the differ-ential pressure between point A andthe atmosphere (OA). The calcula-tion of this percentage number isshown as an example in Fig. 10.

The authority of the RA damper isthe differential pressure across thedamper, when it is fully open as apercentage of the differential pres-sure between points A and B.

The authority of the EA damper isdetermined by the fully open differentialpressure and the differential pressurebetween point B and the atmosphere.

Unfortunately, most dampers arepoorly matched. See Fig. 11. Thisresults in a non-linear OA/RA mixingratio, a variable combined flow (MA)and pressure variations in the mixingplenum, which may affect the buildingpressure.

III. ADDITIONAL CONSIDERATIONSIII-A. Economizer Systems

Most economizer systems have one outside air (OA), one return air (RA) and one exhaust air (EA) damper. The dampers are usual-ly operated in unison, so as the OA and EA dampers close, the RA damper opens. The dampers should be sized so they compli-ment each other, so an increase in the OA flow is matched by an equal decrease in the RA flow. Now, the total flow should be con-stant, regardless of the mixing ratio between OA and RA flow, and the resistance to the flow will be constant, so the pressure in the

6


0 10 20 30 40 50 60 70 80 90 100 % OA/RA MIXING RATIO

FLOW

%

100

90

80

70

60

50

40

30

20

10

0

Ideally the combined flow MA (or SA) should remain constant, regardless of the OA/RA mixing ratio.

TOTAL FLOWMA

RA

OA

IDEAL FUNCTIONFig. 9 - Ideal Function

ACTUATORS

EA

OA

RA

SA

RA

B

A MA

Fig. 8 - Economizer System

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mixing plenum will stay constant.Unfortunately, this is extremely hardto accomplish, because the dampershave a non-linear characteristic.There are also other considerationsthat have to be taken into account,when the dampers are sized andselected.The indoor air quality is very depen-dent upon the amount of fresh out-side air that is introduced into thebuilding. The economizer varies themixing ratio of outside and return airto meet the varying conditions. At100% outside air, maximum freecooling is provided, and when theload changes, the mixing ratio ischanged until the outside air volumeis reduced to a specified minimumvolume which satisfies the indoorquality requirements. In many sys-tems, the economizer is at the mini-mum position, and provides the mini-mum volume of outside air whenthere is a need for mechanical cool-ing or heating. At intermediate load,the mixing ratio is changed to main-tain a constant mixed air (MA) tem-perature.

Minimum OA CFM can be set a num-ber of ways.

A pitot-tube traverse is usually themost accurate if a 100% effectiveduct length is available to get avelocity pressure profile uniformenough for good readings.

Totalling outlet CFM is possible buterror due to 5 - 15% duct leakageoccurs.

As a check to one of the abovemethods, the following formula canbe used: TMA = TOA (%OA) + TRA(%RA); where TMA = mixed air temp.,TRA = return air temp., TOA = OutsideAir Temp., % OA = % OA volume, %RA = % RA volume. If % OA =10%,then % RA = 100-10 = 90%; TOA =60; TRA = 70; then TMA = 69within the error range, 2, of thesensors. It is important to use accu-rate temperature sensors.

Some close coupled equipment(rooftop units in particular) are diffi-cult to control properly. The lack of areturn fan or exhaust often leads toOA entering from the so-called EAdamper until the RA damper is nearly

7

FAN

FAN

EA

OARA

SA

A = MIXING PLENUM

PT

PD

PD

PD

PT

PT

A

B

Authority = Pressure drop over damperTotal pressure drop = x 100%PTPD

PD = .01 in. W.G.PT = .125 in. W.G.

Example: OA

Authority = =x 100%.125.01 8%

OA

CALCULATION OF THE AUTHORITY

0 10 20 30 40 50 60 70 80 90

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

DAMPER POSITION, DEGREES OPEN(OA/RA Mixing Ratio)

PERC

ENT O

F MAX

IMUM

FLOW

OA

1%OP

POSE

D BLA

DE

MA

COMBINED FLOW

RA 10%

OPPOSED BLADE

Fig. 10

Fig. 11

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closed. The building overpressurizesduring most of the year.

The minimum outside air volume isaccomplished by limiting the modula-tion of the dampers to a minimumposition which provides the desiredoutside air volume. It is very impor-tant that this position is controlledaccurately because a small position-ing error will have a disproportionate-ly large effect upon the minimum out-side air volume. An alternative solu-tion is to use two OA dampers. Oneis modulated in unison with the RAand EA dampers, as describedabove. The other damper is smallerand is opened when the fan is run-ning. This damper supplies the mini-mum OA volume. See Fig 12.

III-B. Building Pressure

No building is completely t ight.Therefore a slight positive pressureis required, in order to prevent anyinfiltration of outside air into thebuilding. Leakage of air into thebuilding can cause severe problemswith the indoor climate and the build-ing structure, especially in the heat-ing season.

The pressurizing of a buildingdepends upon a number of factors,

and the economizer is just one of them. However, if the economizer system can not maintain a constant pressure in the mixingplenum (Point A in Fig. 12), the pressurization of the building will be affected.

III-C. Mixed Air Temperature Control

The mixed air temperature control is accomplished by varying the mixing ratio between the outside and return air. There are manythings that can complicate this function and cause stability problems.

If the dampers are poorly matched, the mixing ratio will not change as a linear function when the dampers are operated. An addition-al complication is that the pressure in the mixing plenum will not stay constant. The result of this is that at some portions of thedamper movement, there will be a large change in the mixed air temperature, while at other portions the change will be quite small.This non-linear function causes stability problems for the mixed air temperature control.

III-D. Actuators - General

When the fans are running, the dampers are subject to forces which increase friction. The resistance created by this friction must beovercome by the actuators. There are also dynamic forces that act upon the damper blades, and tend to turn them either clockwiseor counterclockwise, depending upon the position of the blades. The actuators must be able to overcome these forces to positionthe dampers accurately. See Fig. 17.

Usually, the actuators respond to a variable analog control signal, which either is pneumatic (315 psi) or electronic (210 V).

Ideally the dampers should be repositioned in direct proportion to the control signal, and follow a Nominal Signal/Position Curve.This is usually not the case, because of the hysteresis in the actuators and linkage.

8


Fig. 12

ACTUATORS

EA

OA MA SA

RA

B

A

Min. OA On/Off

DAMPER 2

DAMPER 1

MODULATING

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III-E. Hysteresis

See Fig. 13. All actuators havesome hysteresis, which means thatthere is one position/signal curve forincreasing signals, and a slightly dif-ferent curve for decreasing signals.The result is that the position will beslightly different if it is an increasingor decreasing signal.

The control signal has to be changedby a certain value in order to reversethe movement of the actuator. Thisdead zone is the hysteresis, andoften is expressed as a percentageof the control signal range. Mostactuators are connected to thedampers via a linkage, which has aninherent play (slack). The actuatorcan move back and forth a few per-cent before the damper begins tomove. This is also a form of hystere-sis, which adds to the hysteresis ofthe actuator, so the total hysteresiscan be quite large.

The hysteresis creates two seriousproblems:1. It adds to the stability problems of

the mixed air temperature control.2. It makes the positioning of the

actuators very inaccurate. This isespecially serious with respect tothe minimum position of the out-side air damper. If the controlsignal is increased from zero(closed OA damper) to the valuethat represents the Min. positionof the OA damper, a lower thandesired Min. position will result. If the control signal is decreased from a high value to the value that represents the Min. positionof the OA damper, a higher than desired Min. position will result. This difference in the Min. position, depending upon if it hasbeen approached from above or below, will have a very large impact upon the Min. outside air volume. See Fig. 16.

The hysteresis must not be confused with the Resolution. Resolution is the smallest increment the actuator will move, when fineand slow changes in the control signal is made.

The fact that in some applications, actuators can modulate the dampers very smoothly, as long as the control signal is continuallychanged in the same direction, is not any proof that there is no hysteresis. The hysteresis will only reveal itself when the movementof the damper is reversed. Therefore the hysteresis should be measured by first increasing the signal to, for example, 40%, andthen decreasing the signal very carefully, until the damper (not only the actuator) just begins to move. The difference in the signal isthe hysteresis, so if the damper begins to close at 25% signal, the hysteresis is 40 - 25 = 15%. Of course, this test must be donewith the fans running, applying forces to the damper which the actuator must overcome. Hysteresis should be measured at differentpoints of damper movement, with a special emphasis on the Min. OA position.

III-F. Accuracy

The combination of poorly matched dampers and total hysteresis in the linkage and actuators can make stable control of mixed airtemperature very hard to accomplish, even if advanced controllers are used.

If the minimum position of the OA damper is not controlled accurately, the Min. OA volume will be smaller or larger than the required

9

Fig. 13

0 10 20 30 40 50 60 70 80 90 100 % SIGNAL

ACTUATORPOSITION

%

100

90

80

70

60

50

40

30

20

10

0

Hysteresis = 20%

NOMI

NAL S

IGNAL

/

POSIT

ION C

URVE

3 4 5 6 7 8 9 10 11 12 13 PSI

Decre

asing

Increa

sing

A

BC

D

EXAMPLE: Point A The signal is 70%, and the position is 60%.Point B The signal is increased to 100%, the position is 90%.Point C The signal is decreased to 80%, the position remains at 90%.Point D The signal is decreased to 70%, the position will be 80%.

The signal is the same in points A and D, but the position is different, 80% instead of60%. The resulting actuator position will be different, depending upon whether it is con-trolled by an increasing or decreasing control signal.

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value. A lower volume will result in apoor indoor air quality. A higher vol-ume will result in needlessly highoperating costs.

III-G. Actuators - Pneumatic

Pneumatic actuators are so commonthat we should study their function indetail.

They are reliable, have spring returnfunction, but usually operate with avery high hysteresis. This is inherentto the design. See Fig. 14. Thepressure of the control signal acts onthe diaphragm and produces a force,which pushes out the stem and com-presses the spring until the forcesare in balance. The position of theshaft will change in proportion to thecontrol signal. (The nominal sig-nal/position curve).

However, in order to produce a netforce at the output shaft, there mustbe an imbalance between thediaphragm and the spring. The forcefrom the diaphragm must be strongerthan the spring in order to produce apushing force, and the force from thediaphragm must be less than thespring force in order to produce apulling force. This deviation betweenthe posit ion and the nominalsignal/position curve is called springrange shift.

Fig. 15 shows a pneumatic actuatorwhich is required to produce a clock-wise (+) 50 in-lb torque to open adamper, and a counterclockwise (-)50 in-lb torque to close it. The mini-mum effective radius of the linkagearm is 2". This means that the push-ing or pulling force of the actuator is+25 lb. or - 25 lb. (Torque/Effectiveradius = Force. 50/2 = 25 lb.)

The outside diameter of this actuatoris 7 1/2", which corresponds to aneffective diaphragm (piston) area ofapproximately 33.3 sq. in.

When the signal pressure and theposition are in balance, no net forceis produced at the output shaft. Thisis represented by the nominal sig-nal/position curve. In order to pro-duce a 25 lb. force the signal pres-sure has to be higher or lower thanthe nominal signal/position curve.

10


Fig. 14

SIGNALFA

FSFL

FRSPRING

DIAPHRAGM

BALL JOINT

BLADE

SHAFT

LOAD

SIDE SEALS

STEM

FA = FS + FR + FLFA = Air Signal (Power) PositiveFS = Spring PositiveFR = Resistance and Friction Always Resists ChangeFL = Damper Load Negative and/or Positive

For any value of FA (Signal) the actuator takes a position dependent on the forces acting onit. Repeatability is low. The hysteresis is high. With age, FR increases. 1.5 PSI hysteresisis normal over a 5 PSI span.

Fig. 15 - Typical Pneumatic Spring Range Shift and Hysteresis

0 10 20 30 40 50 60 70 80 90 100%

14

13

12

11

10

9

8

7

PSI A

IR S

IGNA

L

8.75

POSITION

7.25

MIN EFFECTIVERADIUS = 2"

90

REQUIRED TORQUE = 50 in-lb

EFFECTIVE DIAPHRAGMAREA = 33.3 SQ. IN.

PNEUMATIC ACTUATOR

7 1/2"OUTSIDEDIAM.

FORCE

NOMINA

L SIGNA

L /POSI

TION C

URVE

SPRING

RANGE

8 - 13 P

SI

+ 50 IN-

LB

50 I

N-LB

0 IN-LB

Resultant Pressure Variation to change the direction of the torque from +50 to 50 in-lb: 8.75 - 7.25 = 1.5 PSI Hysteresis = 1.5 PSIHysteresis as a percentage of the spring Range:

1.5 x 100 = 30% HYSTERESIS =30%!13 - 8

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The resultant pressure variation is 25lb./33.3 sq. in. = 0.75 psi. Therefore,in order to change from +50 in-lb to-50 in-lb torque, the total variationhas to be 2 x 0.75 = 1.5 psi. This is30% of the spring range of a typicalpneumatic actuator (8 to 13 psi).

The hysteresis and spring range shiftin an actual installation dependsupon how generously the pneumaticactuator is sized, in comparison tothe required torque. If the actuatorsare oversized, the hysteresis will beless. The recommendation of thepneumatic actuator manufacturers istypically 1 1/4 - 2 in-lb for eachsquare foot of damper area. This isless than adequate. The resultinghysteresis can be 12% or more. SeeFig. 16.

III-H. Positioner

The accuracy of pneumatic actuatorsis considerably improved if they areprovided with positioning relays(positioner). A positioner compares the actual position of the actuator with the control signal, and changes the pressure to thediaphragm until the actuator assumes a position that corresponds to the control signal. This reduces the hysteresis to about 1/4 psi,which can be 2 1/2 - 5%, depending upon the operating range. However, frequent recalibration is needed to maintain the accuracy.

The additional cost of the positioner, and its adjustment, must not be forgotten when comparisons are made between pneumatic andelectric actuators.

III-I. Torque

The torque that is required to operatea damper depends upon the size,type, quality and condition of thedamper. It is also dependent uponthe differential pressure and air flow.Contrary to popular belief, the maxi-mum required torque is not always atthe closed position. Typically, themaximum torque requirement isfound at about 30% open position.See Fig. 17.

III-J. Linkage

The actuator is connected to thedamper via a linkage, which has acouple of ball joints, pivots and otherelements that have some play. Theslack in the linkage can easily causea 1 - 3% hysteresis, when the stresschanges from a pulling force to a pushing force. This effect requires the actuator to move about 1 - 3% before the damper begins tomove. The amount of hysteresis depends upon the condition of the linkage and how well it has been adjusted. If the linkage isimproperly adjusted and the joints are worn, the hysteresis can actually be larger than 5%.

11

Fig. 16 - Results of Hysteresis on Min OA Air Flow

10 20 30

10

20

30

0

VARIATION50 - 150%

OF DESIREDMIN. OAVOLUME

HYSTERESIS = 12%

OAFLOW

%

% OA DAMPER POSITION

*If the authority is less,the variation will be even larger.

Detail of an Opposed Blade Damperwith an authority of 10%*

MIN. 10% OA FLOW DESIRED5 - 15% RESULT

CONTROL POINT = 6 OF DESIRED MIN.OA DAMPER POSITION

Fig. 17 - Typical Torque Requirement

0 10 20 30 40 50 60 70 80 90

100

90

80

70

60

50

40

30

20

10

0

MAXIMUM

DEGREES, DAMPER ROTATION

PER

CENT

O

F M

AXIM

UM T

ORQ

UE

NEGATIVE POSITIVE

Actual torque needed varies with blade type, pressure drop, system effects, etc.

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III-K. Actuators - Electronic

Electronic proportional actuatorsrequire both a power source, usually24 VAC, and a control signal, usually0 to 10 VDC or 4 to 20 mA. Theiroperation can be compared to apneumatic actuator with a positivepositioner. However, unlike its pneu-matic counterpart, the electronicactuator requires no field calibrationand usually no field maintenance.The position feedback is more pre-cise than a pneumatic devicebecause of a repeatable, geared,interface between the actual actuatorposition and its feedback monitoringsystem. The feedback signal is alsousually available as an output fromthe actuator to either monitor theactuator position or as signal for partof a control sequence.

Feedback can be generated by sev-eral methods. Fig. 18 and Fig. 19show two of these methods. Fig. 18shows a typical potentiometer circuitused to measure feedback. In thiscase, the signal from the potentiome-ter is fed into a differential amplifieralong with the input signal. The dif-ferential amplifier looks at the differ-ence between the input and feed-back signal. It then gives a signal tomove the actuator clockwise or coun-terclockwise until the feedback signalmatches the input signal. Fig. 19shows a newer method which usesmicroprocessor technology. In thiscase, a microprocessor communi-cates to an application specific inte-grated circuit (ASIC). The ASIC bothcontrols and monitors a brushlessDC motor. By monitoring the digitalpulses generated at the brushlessDC motor, the exact position of theactuator can be determined by themicroprocessor. The microprocessoralso allows for special control criteriato be used in either operational char-acteristics or input signal processing.

III-L. Electronic Actuators - Conventional Crank Arm Type

Conventional electronic and electrohydraulic actuators typically have a small hysteresis of about 1%. These actuators are mountedto dampers in a similar manner as a pneumatic actuator, by the use of a linkage. This, as in the case of pneumatics, adds 1% to 3%of hysteresis to the system and care must be taken in its setup. Electrohydraulic actuators are not gear type actuators; they worksimilarly to a pneumatic actuator by building up pressure against a spring. Because they work against a spring they are subject tospring range shift.

12


Fig. 18

90

0

+

10V

0V

Input Signal210 V

420 mA

Output Signal210V

OutputSignal

Conditioner

InputSignal

Conditioner

FeedbackPotentiometer

mechanical connection with Gearbox

M+

DifferentialAmplifier

Fig. 19

INPUT SIGNAL CONDITIONER

BrushlessDC Motor

MICRO-PROCESSOR

ASICMotor-management

OUTPUT SIGNAL CONDITIONER

OUTPUT SIGNAL

210 VDC

INPUT SIGNAL

210 VDC420 mA

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III-M. Electronic Actuators - Direct Coupled

Direct coupled actuators, just as the crank arm type, have a hysteresis of about 1%. However, this style of actuator does not requirethe use of a linkage. Because of this, the additional hysteresis of a linkage is not present. Without a linkage between the actuatorand damper, the damper position can be controlled more accurately and the actuator torque is transmitted more efficiently to thedamper. Also, the installation time is dramatically reduced. The following table gives a comparison of several actuator/damper com-binations and their relative accuracy.

As can be seen, direct coupled elec-tronic actuators are far more accu-rate than any other type of actuator.

Fig. 20 shows an exploded view of atypical Belimo direct coupled actua-tor mounting. It is easy to see justhow simple and straightforward thismounting method is. Direct coupledmounting greatly reduces installationtime and lowers installation costsbecause fewer parts are needed.

Fig. 21 illustrates the differencebetween direct coupled mountingand mounting using linkage. Noticethat the use of linkage often requiresmounting the actuator on an inde-pendently located mounting bracket.The linkage itself generally consistsof at least two crankarms connectedby an operating rod attached to thecrank arms with ball joints. Linkagesystems can become quite complexand a number of geometrical issuesmust be addressed that can effecttorque and response times. Pleaserefer to Belimos Mounting MethodsGuide, Section 6, for more informa-tion about damper linkages.

It is also possible to mount a directcoupled actuator separately, andconnect it to the damper via a link-age. About 20% of installationsrequire this, but it should be usedonly if it is completely impossible tomount the actuator directly. TheBelimo mounting instructions for link-ages must be followed. If it is an oldinstallation, replace the old ball jointswith new ones that have the leastpossible play. It is important that thehysteresis be as small as possible.

13

Fig. 21

0 1

R

Fig. 20

DAMPER POSITIONING ACCURACYPneumatic Pneumatic Electronic Belimo

Without Positioner With Positioner w/ Linkage Direct CoupledActuator 12%a 2 1/2 - 5%b 1% 1%Linkage 1 - 3% 1 - 3% 1 - 3% NoneE/P Transducer 2% 2% None NoneTotal 15 - 17% 5 1/2 - 10% 2 - 4% 1%

a 30% if undersized. b Before calibration drift

Actuator attachesdirectly to dampershaft with universalmounting clamp.

Position indicator

Damper shaft

Anti-rotationstrap

Rotation reversingswitch

Manual overridebutton

Direct coupledwith universalmountingclamp

Belimodirectcoupledactuator

Crank arm type actuatormounted using linkage

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IV. OPTIMIZING ECONOMIZER SYSTEMSIV-A. Sizing of Dampers

The installed characteristics aredetermined by the authority of eachdamper. By sizing each damper cor-rectly, the installed characteristics ofthe dampers can be chosen in sucha way that they complement each other.

Fig. 22 & 23 show examples ofdampers that are poorly matched. InFig. 22 the OA and RA dampers areof opposed blade type, and havehigh authorities. The result is thatthe total air flow (MA) will not be con-stant. For example, when thedampers are in the mid position, thetotal flow will be much less than nor-mal. In Fig. 23 the dampers are ofparallel type and have a very small(2%) authority. The result is that theflow, when the dampers are in themid position, will be much larger thannormal.

Fig. 24 shows an example where thedampers have been selected andsized so their installed characteristicscomplement each other. The OAdamper has opposed blades, and theRA damper has parallel blades.Both have an authority of 15%. Ascan be seen, the total flow (MA)remains rather constant regardlessof the mixing ratio. As long as thevariation in the total flow (MA) is lessthan 15%, the dampers can beregarded as well matched. Ofcourse, each damper has to be sizedwith respect to the air flow and theavailable differential pressure. TheOA damper should always be sizedfor a larger flow than the EA damper.The differential pressure across theRA damper is larger than across theEA damper, so the RA damper willbe the smallest.

It is important to remember that theremust be a slightly negative pressureat point A, in order for the outside airto enter the building. The pressureat point B has to be slightly positivein order for the exhaust to leave thebuilding.

Very often the dampers are selectedbased upon the available duct size,with little concern for how well theyare matched. In many installations,

14


Fig. 23 - Example of Poorly Matched Dampers

0 10 20 30 40 50 60 70 80 90

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

DAMPER POSITION, DEGREES OPEN (OA/RA Mixing Ratio)

PER

CENT

O

F M

AXIM

UM F

LOW

MAT O T A L F L O W EA

MA

RA

OA

B

A

OA = PARALLEL BLADERA = PARALLEL BLADE

FLOW VARIATION IN A REAL INSTALLATION(VARIABLE DIFF. PRESSURE A B)

OA

RA

2%

AUTH

OR

ITY2%

AUTHORITY

This MA flow variation will only be accomplished under laboratory conditions, when the differential pressure across the economizer (between A-B) is held constant. In a real installation, duct work, coils, filters, fan curve, etc., will reduce the flow variation. Instead the differential pressure (between A-B) will vary, which will affect the building pressure.

CONSTANT DIFFERENTIAL PRESSURE A-B

Fig. 22 - Example of Poorly Matched Dampers

0 10 20 30 40 50 60 70 80 90

100

90

80

70

60

50

40

30

20

10

0


PER

CENT

O

F M

AXIM

UM F

LOW

MA

T O T A L

F L O W

EA

MA

RA

OA

B

A

This MA flow variation will only be accomplished under laboratory conditions, when the differential pressure across the economizer (between A-B) is held constant. In a real installation, duct work, coils, filters, fan curve, etc., will reduce the flow variation. Instead the effect will be that the differential pressure between A-B will vary. Actually, both the flow and pressure will vary.

OA = OPPOSED BLADERA = OPPOSED BLADE

OA

20% AU

THOR

ITYRA

20% AUTHORITY

FLOW VARIATION IN A REAL INSTALLATION(VARIABLE DIFF. PRESSURE A B)

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the OA damper is installed next tothe weather louver, and thereforehas to be the same size to keep thevelocity through the louver below 500FPM to avoid snow and rain enter-ing. This results in a very smalldamper authority and this makes thesizing of the RA damper very impor-tant and demanding.

IV-B. Linearization

See Fig. 6 & 7. If we study theinstalled flow characteristics of adamper at different authorities, wewill find that the bottom part and toppart of the curve is very non-linear.However, between about 10% and80% flow, the characteristics arerather linear. This is true for all curveswith authorities between 1% to 50%.

Most dampers are oversized, sothere is no need to open them fully,in order to get the desired maximumflow. For example, if we limit themaximum opening of a damper to80% flow, the non-linear top portioncan be eliminated.

Outside air dampers must provide aspecif ied minimum OA volume.Therefore, they must not be modulat-ed below a certain minimum position.This is fortunate, because it elimi-nates the lower non-linear portion ofthe curve. Now, we can limit theoperation of the damper to the linearportion only.

See Fig. 25, and the 1% authoritycurve (opposed blade). This damperis operated by a direct coupled actu-ator (2 to 10V). In order to open thedamper to a 20% (Min. flow), the control signal has to be 2.8 V. In order to open the damper to an 80% flow, the control signal hasto be 5.2 V. In other words, it takes only a 2.4 V change in the control signal to get a linear change from Min. to Max. flow. Becauseonly a small portion of the operating range is utilized for this application, the resulting MA temperature can be erratic due to overcontrol.

IV-C. Range Controller

Belimo offers a very useful device, called the SBG24 range controller (see page 18). It accepts the control signal (2 to 10 V) andmodifies it so the damper is at the Min. position, when the control signal is 2 V, and the damper is at the Max. position when the con-trol signal is 10 V. In other words, the full swing of the control signal (2 to 10 V) is utilized to vary the flow from Min to Max.

It is common practice to mechanically link the OA and RA dampers. This will save the expense of one actuator. Unfortunately, thereare a couple of serious disadvantages associated with this. The most obvious problem is that the linkage will add a hysteresis to theoperation of the RA damper. An even more serious problem is that the dampers have to be selected very carefully, so they are wellmatched. Otherwise the mixing ratio will not be controlled in a linear way, the total flow will not be constant and the pressure willvary. Fig. 22, and Fig. 23, are examples of poorly matched dampers. Fig. 24, is an example of correctly matched dampers.

15

Fig. 24 - Example of Properly Matched Dampers

0 10 20 30 40 50 60 70 80 90

120

110

100

90

80

70

60

50

40

30

20

10

0


PER

CENT

O

F M

AXIM

UM F

LOW MA

15% AUTHORITY

15%

AUTH

ORITY

EA

MA

RA

OA

B

A

OA

RA

T O T A L F L OW

OA = OPPOSED RA = PARALLEL

Fig. 25 - Installed Opposed Blade Damper Flow Characteristics

100

90

80

70

60

50

40

30

20

10

0

1%2%

3%4%

5%

10%

15%

20%30%

50%

DAMPER POSITION, DEGREES OPEN

PER

CENT

O

F M

AXIM

UM F

LOW

2 3 4 5 6 7 8 9 10V


CONTROL SIGNAL

100%

0 10 20 30 40 50 60 70 80 90

2.8V 5.2V

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By using the range controller, theoperation of the dampers can bemodified so a more linear portion ofthe dampers are used. This alsoallows poorly sized dampers to beoperated so they can more accurate-ly complement each other. The fullrange of the control signal is utilized,and there will be a linear relationshipbetween the OA/RA mixing ratio andthe control signal. Also, the pressurein the mixing plenum will be con-stant.

See Fig. 26. Each of the dampershas its own actuator, so they can beoperated independently of eachother. The OA and EA damper actu-ators are connected to the samerange controller. The RA damperactuator is connected to a separaterange controller.

It is possible to duplicate the functionof the range controller with the software of a DDC system. This is acceptable, but it is not necessarily a cost saving, because anadditional analog output is needed. Another problem is that the fine tuning of the range controller should be done in conjunction withthe balancing, and it is hard for the balancing contractor to access the DDC controller.

For a detailed description of the Range Controller, see the catalog sheet for SBG 24.

IV-D. Choice of Dampers

Fig. 27 shows an economizer withOA and EA dampers of opposedblade type. The RA damper has par-allel blades, mounted so the air flowis deflected up against the OAdamper for good mixing. (This is onlyone of a number of possible destrati-fication methods.)

Because in many installations, theOA damper has the same size as theweather louver, it will be oversized incomparison to the flow, which resultsin a low authority. Opposed bladedampers have characteristics thatcan control small air flows moreaccurately than parallel bladedampers. For example: in order tosupply a 20% flow, an opposedblade damper, with an authority of

1%, has to open to an 8 position, while a parallel blade damper, with the same authority, should open to a 2.5 position. See Fig. 28and Fig. 29. If the total hysteresis in the actuator and linkage is just 5, the parallel blade damper may supply anything from 0% to46% minimum OA volume (0 to 7.5 damper position). The positioning of an opposed blade damper is not quite as critical. A 5error will result in a 8 to 30% deviation from the desired OA volume (3 to 13 damper position), so the opposed blade damper is abetter choice when the OA damper is oversized and has a small authority. See also Fig. 16. A direct coupled actuator which has a1% hysteresis will result in only a 2.5% deviation from the desired OA volume.

The range controller has the capability to adapt the control signal so the dampers are operated in such a way that they complementeach other. This is done by limiting the damper movement to the linear portion only, and it makes the task of sizing the dampers less

16


Fig. 27

EA

OA

RA

SAMA

Opposed Blade

Parallel Blade

RA

Fig. 26

ACTUATORS

EA

OA

RA

SA

CONTROL SIGNAL

MA

RangeController I(Direct Acting)

RangeController II(Reverse Acting) 210 V

7.2

to 2

.0 V

*

2.8 T

0 5.

2 V

(See F

ig. 25

)

RA

* Range given for example only.

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critical. However, this does notmean that it no longer matters howthe dampers are sized. If thedampers are correctly sized, therange controller only needs to makeminor corrections, and a large por-tion of the damper movement can beutilized. However, if the dampers arepoorly matched, a larger correctionhas to be used, and only a small por-tion of the damper movement can beused. The range controller will allowa great latitude when sizing thedampers, and even if the dampersare poorly matched, the function willbe improved. However, the bestresult is achieved if the dampers arenot excessively oversized.

V. ADVANTAGESV-A. Direct Coupled Actuators

Electronic direct coupled actuators offer a tremendous advantage in that they are very accurate, and can position the dampers withthe smallest possible hysteresis. If used in conjunction with the Range Controller, the dampers can be operated at a much superiorlevel of control than would otherwise be possible.

V-B. Range Controller

The Range controller offers the following advantages: The sizing of the dampers is simplified, because the dampers need not be perfectly matched. The mixed air temperature control is improved. The Min. OA position is set in the range controller. The total flow can be (balanced) adjusted by the range controller. Because the dampers are operated so they complement each other, the total flow will remain constant

regardless of the mixing ratio. The pressure in the mixing plenum will remain constant. The range controller can be mounted adjacent to the economizer, so balancing is simplified. Flow measure-

ments and adjustments can conveniently be done at the same location.

VI. MODERNIZATION OF OLD INSTALLATIONSWhen an old installation is modernized with a DDC system, it can be tempting to keep the old actuators if they still are working. Thisis always a very unfortunate decision, because of the poor accuracy offered by conventional actuators. The full benefits of the DDCsystem will not be realized, because the dampers cannot be controlled accurately.

Most existing installations have problematic damper control, which would benefit tremendously if the old actuators were replacedwith direct coupled actuators and range controllers, and a recommissioning of the OA CFM is performed.

VII. SUMMARYThis guide has introduced a number of concepts, but is not a complete discussion. The importance of sizing and matching thedampers have been explained, with respect to the authority and other control aspects. However, for the final calculation and sizingof the damper, we refer to the damper manufacturers tables, charts and instructions.

The range controller simplifies the sizing, and makes it possible to adjust for the as built conditions.

Finally, it cannot be emphasized strongly enough that the accurate and hysteresis free operation of the direct coupled electronicactuators is of the greatest importance for the economical operation of air handling systems.

17

Fig. 28 Fig. 29

0 10 20 30 40

60

50

40

30

20

10

0

DAMPER POSITION DEGREES OPEN

PER

CENT

O

F M

AXIM

UM F

LOW

1%

3%

5%10

%

15%20%

30%50%

100%

2.5 7.5}}

+55Hysteresis

PARALLEL BLADE DAMPERINSTALLED FLOW CHARACTERISTICS AT

DIFFERENT DAMPER AUTHORITIES (1100%)

OPPOSED BLADE DAMPERINSTALLED FLOW CHARACTERISTICS AT

DIFFERENT DAMPER AUTHORITIES (1100%)

+55Hysteresis

60

50

40

30

20

10

0PER

CENT

O

F M

AXIM

UM F

LOW 1%

3%

5%

10%

20%

50%100%

0 10 20 30 40 DAMPER POSITION

DEGREES OPEN3 8 13

8

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18

Wiring diagram

ApplicationThe SBG24 range controller is an interface device usedbetween controller and damper actuator. It is used to compen-sate for the non-linear control characteristics of dampers dueto their damper authority.

OperationThe SBG24 is designed to rescale the control output to theactuator. The maximum and minimum damper position can beset in the SBG24. The controller output signal (2 to 10 VDC, 4to 20 mA, or 0 to 10 V phasecut) then modulates the actuatorbetween the maximum and minimum set limits. By only modu-lating between these set limits, the damper typically can becontrolled more in the linear portion of its damper authoritycurve. By the use of a SBG24 on both the outdoor air andreturn air dampers in an economizer system, a single con-troller output can more accurately position these dampers sothe change in flow of one damper is off-set by an equal changein the other.

Overrides can also be used to make the dampers go to eithera fully open or closed position.

Note: For an explanation of damper authority, see BelimoDamper Application Guide 5.4.

2

4

24 VAC Transformer

U 2 to 10 VDC Feedback signal

0 to 20 V Phasecut

Line Volts

1 Common2 + Hot

3 Y14 Y25 U 6 Z

1 Common2 + Hot

3 Y1

5 U

1 Common2 + Hot3 Y1

5 U

()(+)

Control Signal0 to 10 VDC

2

1

AB

C

OverrideSwitch Position

A = ClosedB = Normal

C = Open

-SR US

SBG24

I

N

P

U

T

OUTPUT

3

5

2

1 Provide overload protection and disconnectas required.Actuators may be connected in parallel.Power consumption and input impedancemust be observed.The actuator and SBG-24 may be pow-ered from the same transformer.The controller should be powered from aseparate transformer.

Wire No. 4 on the NM24-SR US is used forfeedback.

3

4

5

Dimensions

Technical Data SBG 24 Power supply 24 VAC 20% 50/60 Hz Power consumption 1W without actuator Transformer 1.5 VA without actuatorElectrical connection terminalsControl signal Y Y1 0 to 10 VDC Y2 0 to 20 V phasecutInput impedance 100 k (0.1 mA) 8 k (50 mW)Operating range 2 to 10 VDC 2 to 10 V phasecutOutput signal 2 to 10 VDC (adjustable) max .5 mADirection of rotation reversible with switch A/BPositioning range adjustable max. = 0.2 to 1 (approx. 20

to 90 rotation angle)min. 0 to 80% of max.

Measuring voltage U 2 to 10 VDC (max 0.5 mA) for position0 to 1

Ambient temperature -4F to +122F [-20C . . . +50C]Storage temperature -40F to +176F [-40C to +80C]Housing NEMA type 2 Housing rating UL94V-0 (flammability rating) Quality standard ISO 9001Weight 14 oz [400 g]

Range Controller SBG 24

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Notes/Work Pad

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C200

59-08

/00-

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Belim

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Actuators for heating, ventilation, air-conditioning

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Air Damper Actuator Documentation

OverviewDoc. 1.0 Product Guide and

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Belimo Aircontrols (USA), Inc.Corporate Headquarters/ManufacturingDistribution/Customer Service CenterLatin American Customer Service43 Old Ridgebury Road, P.O. Box 2928Danbury, CT 06813(800) 543-9038 (203) 791-9915Fax (203) 791-9919Toll free fax order line: 1-800-ACTUATE (1-800-228-8283)

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VOL 1.0 1998

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